This invention relates to graphical data apparatus and, more particularly, to an apparatus for locating the position of a movable element in a data space.
Graphical data systems have been employed for some time to determine the position of a movable element or stylus in a defined coordinate system. Earlier systems employed a stylus in the form of a writing pen or pointer mechanically coupled to a set of arms for translating the movement of the stylus into a sequence of usable information signals. Such arrangements proved generally unsatisfactory in that they presented undesirable frictional and inertial limitations.
Subsequently developed graphical data systems, called "digitizers", utilize tablets which have imbedded conductors. These systems, which involve the considerable expense of tablet manufacture, are satisfactory for some uses but are inflexible in that they are limited to applications where a tablet can be employed.
In the U.S. Pat. Nos. 3,838,212 and 3,731,273 there are disclosed graphical data systems which utilize a stylus that generates periodic sound waves and microphones which define a coordinate area. Circuitry is provided which measures the transit time of the sound waves between the stylus and the microphones, and the coordinates of the stylus are determined from the transit times which are translatable to distances since the speed of sound in air is essentially a known quantity. These types of systems are presently in commercial use and offer a distinct advantage over conventional data tablets since no tablet is required and the microphones can be positioned around any desired work space. Operation is considered satisfactory but there are aspects of these types of systems which could stand improvement. For example, the stylus in such systems is "active" and generally requires wire connections for energizing the sound source. In addition to the effects of the wire on manual dexterity, the sound source, typically a relatively high voltage spark, can cause interference in some instances; e.g., where the data area corresponds to an electronic display like a CRT or a gas discharge display.
In the co-pending U.S. patent application Ser. No. 608,917, now U.S. Pat. No. 4,012,558 assigned to the same assignee as the present application, there is disclosed a digitizer apparatus which employs a passive movable element (i.e. a stylus or "puck") which is movable in a data space. First and second spaced receivers are provided and sound waves are periodically generated at a source position spaced from the passive movable element. Timing means, coupled to the receivers and synchronized with the generation of the sound waves, measure the transit time of the sound waves reflected off the movable element to the receivers. The instantaneous rectangular coordinates of the movable element are automatically computed from the measured transit times. In a disclosed embodiment in the referened patent application, a pair of cylindrical microphones are employed to effectively serve as "point" receivers.
The present invention is directed to an apparatus of the type disclosed and claimed in the referenced application, and is an improvement thereon.